One thing I have noticed is many ppl do not realise that by default a copy of your PM is saved in the "Sent folder" unless you uncheck "Save a copy of sent messages in my Sent Items folder by default" (On VBB sites that have that option) and it fills up before ya know it!!

I have it UNCHECKED here on all other VBB sites im on that have the option to do it (Otherwise I manually uncheck the box to save it)